Paul isn’t advocating stupidity in this verse. What he is saying, and this means a lot coming from someone as greatly educated as he, was that knowledge of the Spirit is given by the Spirit. In a community of Romans and Greeks, this must have rankled a few. Greeks especially valued their education. While education can teach us about God, only the Spirit can allow us to know God. Waiting for God to reveal himself, putting ourselves in his hands, is not a process that can be taught by merely reading about it.